Liquid catalyst for crosslinking with an amino resin

ABSTRACT

The invention provides a liquid catalyst capable of cross-linking amino resins. The liquid catalyst has improved flexibility and adhesive properties. It is comprised of a cross-linkable polyvinyl acetate, an acid, and an ammonium salt.

FIELD OF THE INVENTION

This invention relates to the field of liquid catalysts and moreparticularly to liquid catalysts capable of cross-linking amino resins.

BACKGROUND OF THE INVENTION

Amino resins are used in the area of wood products includingparticleboard, hardwood, plywood, decorative laminate, furnitureassembly, etc. Amino resins are supplied in liquid form and are combinedwith a catalyst at the time of use to form an adhesive substance that isapplied to the wood product. This adhesive substance provides adhesionbetween wood surfaces.

The catalysts are either acidic substances or other materials capable ofliberating free acid upon addition to the amino resin. The acidicproperty of the catalyst promotes cross-linking amino resins andaccordingly, the cross-linking or setting of the adhesive substance sothat the adhesive substance has the necessary high strength, waterresistance and cold flow resistance in order to meet its necessaryapplications. For example, hardwood floors used for flooring in trucktrailer, truck bodies and shipping containers are laminated using acatalyst/amino resin with high strength cross-linking. The quality oflamination can then be verified using Fruehauf Engineering Standards,with one of the most important criteria being the wet shear strength.

The amount and type of catalyst used determines whether the adhesivebond can be cured at room temperature or at higher temperatures. Curetime may vary from a few minutes at high temperatures to a few hours atroom temperature. In an application where high frequency heating isrequired, as for example with laminated pieces of wood flooring, it iscommon to reduce the time required to cure the adhesive substance.

Current catalysts used in combination with amino resins are in powderform.

A major drawback with a powder catalyst is that powders are not easy tomanipulate. Mass transfer of powder materials requires special equipmentand/or more human manipulations.

Also, powder catalysts pose certain problems in terms of dust formationduring transfer, which may cause toxic effects on humans (irritatingmucous membranes and upper respiratory tract).

Care must always be taken to mix homogeneously the blend of powdercatalyst and liquid amino resin to avoid any lumping of powder, whichwill cause a bum or an arc if high frequency heating is used.

While the use of a high level of powder catalyst imparts lower settingtime, the amount of powder catalyst mixed with the amino resin islimited to about a maximum of 15% by weight. Increasing the amount ofpowder catalyst above 15% by weight increases the viscosity and reducesthe pot life of the mixture, but decreases the adhesivity and strengthof the mixture. Moreover, powder catalysts do not contribute anyadhesive property to the mixture and a higher concentration of powdercatalyst reduces the relative amount of amino resin in the mixture andtherefore reduces the adhesive performance of the resulting mixture.

When properly cured, the resulting bond of the amino resin cross-linkedwith a powder catalyst is hard and brittle and is characterized by a lowimpact strength and low elongation point prior to breakage.

Accordingly, there remains a need to develop a catalyst that iscommercially beneficial for the wood glue industries and which can beeasily mixed at a relatively high concentration with a liquid aminoresin to provide a resulting adhesive substance with an adequateviscosity, pot life, and setting time. There also remains a need toprovide a catalyst that reduces the cycle time of a high frequencyheating press, while contributing adhesive properties to the finaladhesive substance.

SUMMARY OF THE INVENTION

It is an object of one aspect of the present invention to provide acatalyst with improved adhesive properties and flexibility.

It is a further object of one aspect of the present invention to providea catalyst with improved cross-linking capabilities and properties thatfacilitate mixing with an amino resin.

According to one aspect of the present invention, there is provided aliquid catalyst for cross-linking an amino resin, said liquid catalystcomprising between 52 and 98% copolymer, between 3 and 14% acid andbetween 4 and 14% ammonium salt.

In a preferred embodiment of the present invention, the liquid catalysthas several advantages over currently used powder catalysts, some ofwhich are the following. The liquid catalyst has a relatively lowviscosity that allows it to be easily transferred and mixed with anamino resin. The liquid catalyst provides a fast curing time when curedwith high frequency heating. It provides a flexible glue line with highimpact strength and a high elongation point. The liquid catalystprovides adhesive properties to the catalyst/amino resin mixture and itprovides excellent bond strength and excellent water resistance to theadhesive substance.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a liquid catalyst capable ofcross-linking an amino resin. The liquid catalyst is a compositioncomprised of a copolymer, acid, and ammonium salt. In a preferredembodiment, the liquid catalyst has a storage life of a minimum of threemonths.

In a preferred embodiment of the present invention, the liquid catalystcomposition comprises the following components (all percentages are inweight % to a total of 100%):

Copolymer emulsion > 52% to 98% Acid 14% to 0.01% ammonium salt 14% to2% ionisable salt  3% to 0% inert filler  1% to 0% adjusting water 16%to 0%

(A) Copolymer:

The copolymer is the primary component of the liquid catalyst. It ispreferably present in an amount ranging from about 52 to 98 weight %.Most preferably, it is present in an amount of 70 to 78 weight %.

In a preferred embodiment, the copolymer is comprised of vinyl acetateand a comonomer capable of crosslinking. An example of a suitablecomonomer is N-methylolacrylamide, which shows improved crosslinkingcapabilities with polyvinyl acetate.

The polyvinyl acetate and comonomer form an emulsion that isself-crosslinkable. The emulsion functions as a water resistantadhesive, thereby improving the adhesivity and impact strength of theliquid catalyst over the non-adhesive powder catalysts used in the priorart.

A commercially available source of copolymer is the self cross-linkingpoly-vinyl acetate dispersion available under the trade mark P-1036™from Dural, a division of Multibond Inc. This copolymer is a dispersionin water having a solids content of about 51% (51±1%), a pH of 4.0 to5.0, a Brookfield viscosity at 25° C. (spindle 5 at 20 rpm) of 8,000 to12,000 cps, an average particle size of 0.8 μ, and a minimum filmforming temperature of +15.6° C. The copolymer dispersion is non-ionicwith a density of 1.07 g/cm³. It has a stabilization system based onpolyvinyl alcohol.

N-methylacrylamide is purchased at a concentration of 48%. The amount ofN-methylolacrylamide based on the weight of vinyl acetate may vary from3 to 10%.

It will be apparent to one skilled in the art that other copolymers maybe used within the confines of the invention without deviating from theinvention as claimed. For example, a self-crosslinking polyvinyl acrylicor polyacrylic may be used as part of the copolymer dispersion.

(B) Acid:

In a preferred embodiment, the liquid catalyst includes an acid such ascitric acid, commercially available under the name citric acidanhydrous. It is present in the liquid catalyst in an amount rangingfrom 1 to 14 weight %, and preferably in an amount of from 4 to 7 weight% of the liquid catalyst.

The acid is useful in the liquid catalyst because it initiates hardeningof the catalyst-amino resin mixture by improving the cross-linkingreaction with the amino resin.

A commercially available source of citric acid anhydrous is availablefrom ADM Food Additive Division. It has a solubility in water of 162g/100 ml at 25° C., with a maximum water content of 0.5%, and a maximumlead content of 0.5 ppm. Another suitable acid is salicylic acid.

(C) Ammonium Salt:

In a preferred embodiment, the liquid catalyst also includes an ammoniumsalt. The preferred ammonium salts are ammonium salts derived fromstrong acids, such as, ammonium chloride and ammonium sulfate. Theammonium salt cross-links with the amino resin and improves the overallcross-linking of the liquid catalyst. It is preferably present in anamount ranging from about 2 to 14 weight %, and most preferably from 4to 7 weight % of the liquid catalyst.

It has been found that an amount of ammonium salt higher than 14%results in loss of stability. It shortens the pot life of the liquidcatalyst-amino resin mixture and shortens the storage life of thecatalyst.

A commercially available source of the ammonium salt is ammoniumchloride sold by Van Waters & Rogers, which has a specific gravity of1.53 g/cm³, a pH of 5.5 in 1% water and a boiling point of 520° C.

(D) Ionisable Salt:

The liquid catalyst may also contain an ionisable metal salt, such assodium chloride. An ionisable metal salt is required if the resultingadhesive substance will be cured using high frequency heating. Theionisable metal salt is employed to adjust the electrical conductivityof the adhesive substance when high frequency heating is used to curethe adhesive substance.

The ionisable metal salt may be present in an amount ranging from about0 to 3%, and preferably from 1.5 to 3 weight % of the liquid catalyst.It has been found that an amount greater than 3% results in burning andarcing when high frequency heating is used.

(E) Inert Filler:

The liquid catalyst may also contain an inert filler such as nut shellflour or fine hardwood flour. The inert filler may be present in anamount ranging from 0 to 1 weight % and preferably from 0.8 to 1 weight% of the liquid catalyst.

The inert filler thickens the resulting adhesive substance and acts likea water retention agent to hold the adhesive substance in place when itis under heat. Water retention is advantageous to the extent that therapid heating of the resin mix in the glue line when exposed to highfrequency heating will thin, increasing its viscosity. The thinning maytake place faster than the chemical action of the resin setting. In suchcases, the walnut shell flour or the fine hardwood flour acts as asponge to hold the adhesive substance in place.

(F) Water:

The liquid catalyst may also include water from the aqueous dispersionof the copolymer and additional water to adjust the solid content andviscosity of the mixture. Total water content is preferably in an amountranging from 0 to 16 weight %, and most preferably from 10.5 to 14weight % of the liquid catalyst.

(G) Defoamer:

If necessary, the liquid catalyst may also contain defoamer to preventexcessive foaming during mixing of the liquid catalyst with the aminoresin. A commercially available source of a suitable defoamer is NapcoNDW, sold by Henkel.

(H) Buffer:

If necessary, the liquid catalyst may also contain a buffer such astricalcium phosphate for adjusting the pH of the liquid catalyst. Acommercially available source of a suitable buffer is tricalciumphosphate, sold by Albright & Wilson.

Amino Resin:

The liquid catalyst is crosslinked to an amino resin to form theresulting adhesive substance. An amino resin is a resin made from thecondensation of an amine group with a carbonyl compound and an alcohol.The most commonly used amine compounds are urea, melamine,benzoguanamine, glycoluril, cyclic ureas, thioureas, guanidines,urethanes, cyanamides, etc. The most commonly used carbonyl compoundsare aldehydes such as formaldehyde. The most commonly used alcohols aremethanol, ethanol and butanol.

Typical amino resins that are used for cross-linking the liquid catalystinclude urea formaldehyde resins or melamine urea formaldehyde resins.

Commercial resins exist as mixtures of polymers where each polymer iscomprised of many monomers. Each monomer is then a dimer or trimer. Theliquid catalyst works with the amino resin, independently of the ratioof dimers/trimers.

The amino resins are usually supplied in liquid form having a solidcontent of 50-65% and a storage life of about 3 months at 20° C.

Use of the Liquid Catalyst

In operation, the resulting adhesive substance made from the liquidcatalyst of the present invention can be applied with the sameconventional equipment as that used with powder catalysts.

The liquid catalyst is pre-mixed to form a uniform composition. Theliquid catalyst is then added to an amino resin so that preferably a20-25% composition of liquid catalyst by weight is present in themixture.

The combined liquid catalyst and resin are then mixed to create ahomogeneous composition. At this stage, the nature of the liquidcatalyst is advantageous over powder catalysts used in the prior art inthat powder catalysts often form lumps within the adhesive substancewhich results in arcing when the adhesive application is treated in ahigh frequency press.

The resulting adhesive substance is then fed into a roller applicatorwhere it is applied to the wood surface. The wood is then transferred toa high frequency press or hot press for drying and curing.

Typically, a high frequency press is used to laminate pieces of wood foran application such as a truck flooring. High frequency heating providesa method for raising the temperature of materials that are normallyelectrical insulators by placing these materials between metal platesand applying radio frequency voltage. It facilitates coherentelectromagnetic radiation of energy. In general, the temperature of thewood in the high frequency press is 165° F. to 210° F.

The use of a high frequency press is expensive. In general, it takes 165seconds for one cycle of a high frequency press using a powdercatalyst-amino resin mixture. With a liquid catalyst present in a 20-25%weight composition, the time required for high frequency pressing isreduced to 120 seconds per cycle, thereby significantly decreasing thecost of curing.

A hot press is used for applications that do not require lamination onthe edges of the wood. A hot press is suitable for applications such asface lamination which application may be made to plywood.

The liquid catalyst of the present invention is advantageous in theabove processes in that it facilitates mixing with the amino resin. Inaddition, the use of a liquid catalyst, and in particular the additionof polyvinyl acetate within the catalyst pre-mix, improves adhesivityand cross-linking the amino resin. Improved cross-linking decreases thecold flow between molecules and accordingly, improves the strength ofthe catalyst-amino resin mixture.

As shown in Example 1, the viscosity of the liquid catalyst/amino resinmixture is decreased two-fold over that of the powder catalyst/aminoresin mixture. Viscosity, being a measure of resistance to flow, isimportant for maintaining the adhesive properties of the resultingadhesive substance. If the adhesive substance is too viscous, theadhesive will not wet the wood. If the adhesive is too liquid, it willsoak into the wood, thereby also proving ineffective.

In addition, as shown in Examples 2, 3 and 4, the wet shear strength ofthe liquid catalyst/amino resin is improved over that of the powdercatalyst/amino resin used in the prior art. According to the FruehaufEngineering Standards, the minimum required wet shear is 525 psi whilethe liquid catalyst/amino resin combination of the present invention(with 22% liquid catalyst and 78% amino resin) has a wet shear that istwo to three times the minimum strength. The dry shear is also 10%higher than that of powder/amino resin catalysts. Additionally, in mostcases, the dry shear and wet shear provide 100% wood shear failure,which is indicative of excellent shear strength.

In a preferred embodiment, the pot life, or time elapsing between thetime the resulting adhesive substance is ready for use and the time whenthe mixture is no longer useable is at least 45 minutes.

It is recommended that the liquid catalyst be mixed with the amino resinat temperatures between 16 and 30° C. where the composition of thecatalyst is in its liquid state.

EXAMPLE 1

A liquid catalyst of the following composition was mixed with MUF resin9430 from ARC Resin Corporation and was applied onto oak (edgelaminating) and cured with high frequency heating. A standard mixtureusing powder catalyst 9532 and MUF resin 9430 from ARC Resin Corporationwas also used as a standard. P-1036 refers to Multibond'sself-crosslinking polyvinylacetate.

Composition A % by weight P-1036 : 71.5 water : 10.9 sodium chloride :2.9 citric acid : 7.0 ammonium chloride : 7.0 walnut shell flour : 0.7Properties Initial quality Mixture Viscosity of % by weight cps pH geltime cycle press time adhesion MUF 9430:80 2210 4.9 30 sec 125 secExcellent Comp A:20 MUF 9430:75 2100 4.8 29 sec 120 sec Excellent CompA:25 MUF 9430:85 5450 6.6 42 sec 165 sec excellent Powder (standardCAT:15 powder 9532 catalyst)

EXAMPLE 2

A liquid catalyst of the following composition was mixed with MUF resin9430 from ARC Resin Corporation. A powder catalyst and MUfF resin 9430from ARC Resin Corporation was also used as a standard. The dry shearresults indicate the strength of the catalyst/resin composition. P-1036refers to Multibond's self-crosslinking polyvinylacetate.

Composition B % by weight P-1036 : 71.2 defoamer : 0.3 sodium chloride :2.8 citric acid : 7.0 ammonium chloride : 7.0 walnut shell flour : 0.8water : 10.7 mixture ratio % by weight cycle time dry shear wet shearComposition B: 20 125 3180 psi 626 psi MUF: 80 Powder Catalyst: 15 1601800 psi 525 psi MUF: 85

EXAMPLE 3

A liquid catalyst of the following composition was mixed with MUF resin9430 from ARC Resin Corporation. A powder catalyst and MUF resin 9430from ARC Resin Corporation was also used as a standard. P-1036 refers toMultibond's self-crosslinking polyvinylacetate.

Composition C % by weight P-1036 : 75.5 defoamer : 0.4 sodium chloride :1.5 citric acid : 5 ammonium chloride : 5 wood flour : 0.7 water : 11.9mixture ratio % by weight cycle time wet shear Composition B: 22 120 700psi MUF: 78 powder: 15 160 525 psi MUF: 85

EXAMPLE 4

25% by weight of a liquid catalyst of the following composition wasmixed with 75% by weight of MUF resin 9430 from ARC Resin Corporation,resulting in an increased wet shear of 1206 psi as compared to 525 psifor the standard powder catalyst. P1036 refers to Multibond's selfcross-linking polyvinylacetate.

Composition D % by weight P 1036 70.5 defoamer 0.3 sodium chloride 1.4tricalcium phosphate 1.4 citric acid 4.9 ammonium chloride 4.9 walnutshell flour 0.7 water 15.9

What is claimed is:
 1. A liquid catalyst for cross-linking an aminoresin to form an adhesive substance, the liquid catalyst comprising: i)between 52 and 93% of a copolymer, ii) between 3 and 14% of an acid;and, iii) between 4 and 14% of an ammonium salt.
 2. The liquid catalystof claim 1 wherein the copolymer consists of a monomer and a compoundselected from the group consisting of vinyl acetate, polyvinyl acrylic,and polyacrylic.
 3. The liquid catalyst of claim 2 wherein the monomeris N-methylolacrylamide.
 4. The liquid catalyst of claim 3 wherein theN-methylolacrylamide is present in the copolymer in an amount equal to 3to 10%.
 5. The liquid catalyst of claim 2 wherein the amount ofcopolymer is between 70 and 78%.
 6. The liquid catalyst of claim 2wherein the acid is selected from the group consisting of citric acidand salicylic acid.
 7. The liquid catalyst of claim 6 wherein the acidis present in an amount equal to 4 to 7% by weight.
 8. The liquidcatalyst of claim 6 wherein the ammonium salt is present in an amountequal to 4 to 7% by weight.
 9. The liquid catalyst of claim 6 furthercomprising an ionisable metal salt.
 10. The liquid catalyst of claim 9wherein the ionisable metal salt is present in an amount equal to 0.01to 3% by weight.
 11. The liquid catalyst of claim 9 wherein theionisable metal salt is present in an amount equal to 1.5 to 3% byweight.
 12. The liquid catalyst of claim 9 wherein the ionisable metalsalt is sodium chloride.
 13. The liquid catalyst of claim 6 furthercomprising an inert filler.
 14. The liquid catalyst of claim 13 whereinthe inert filler is present in an amount equal to about 0.01 to 1% byweight.
 15. The liquid catalyst of claim 13 wherein the inert filler ispresent in an amount equal to about 0.8 to 1% by weight.
 16. The liquidcatalyst of claim 13 wherein the inert filler is nut-shell flour or finehardwood flour.
 17. The liquid catalyst of claim 6 further comprisingwater in an amount of from 0.01 to 16% by weight.
 18. The liquidcatalyst of claim 6 further comprising water in an amount of from 10.5to 14% by weight.
 19. The liquid catalyst of claim 6 further comprisinga defoaming agent.
 20. The liquid catalyst of claim 6 further comprisinga buffer.
 21. The liquid catalyst of claim 20 wherein the buffer istricalcium phosphate.